Currently, the mod is required on both client and server or Forge will mark the connection as incompatible.

Forge allows registering an extension point to bypass this check, for mods that don't require both sides to be present.

See the docs here for 1.16.x: https://mcforge.readthedocs.io/en/1.16.x/concepts/sides/#writing-one-sided-mods

and here for 1.18.x syntax: https://mcforge.readthedocs.io/en/1.18.x/concepts/sides/#writing-one-sided-mods

This has been implemented for a while now, sorry for the delay.